The fact that you're reading articles on a fuel economy web site says you're probably already aware that speed kills ... mpg. Resisting the urge to drive fast is one of the easiest and most effective methods for seeing big savings.

We've all seen stats from NRCan and the EPA about how much more fuel is burned as speed rises: Driving at 120 km/h instead of 100 uses about 20% more fuel (NRCan); observing highway speed limits can save 7-23% (EPA).

Those are serious numbers. But reading a stat is one thing - seeing it laid out graphically is a real eye-opener. So I set out to collect some hard evidence for the Blackfly: a graph of speed vs. fuel consumption.

Inspiration & examples ...

The inspiration for this data gathering mini-mission came from seeing a number of graphs for various cars, including this simulated plot for the Prius II. It's an interesting shape that reveals a lot about the car that made it:

The benefit of electric assist is obvious up to around 42 mph, at which point the engine spins up, mileage abruptly falls off a short cliff, and then continues to decrease more gradually as speed rises. The shape of the graph could also have only come from a car with a continuously variable transmission - other than the transition where the internal combustion engine spins up, there are no obvious "steps" in the plot.

Note: that's a calculated graph, with values based on 68F, at sea level, no A/C or wind.

A more extreme example of this kind of test was published earlier this year by Auto Bild magazine. They ran a number of cars at much higher (autobahn) speeds - between 50 and 140 mph on a test track while recording fuel consumption:

1.7 km long runs on the almost level "test course". Each point on the graph is the average of a bi-directional run using cruise control (speed was set in advance of passing a start marker where the ScanGauge was reset).

I used to be able to set the cruise control in my 1989 Honda Accord at 95 km/h (59 mph) and exceed the car's highway EPA rating by about 20%.

On the same day I ran the Firefly test, I took a relative's 1997 Toyota Camry automatic down the same test road; it achieved its 30 mpg EPA highway rating at 115 km/h (71.5 mph). Quite a difference.

The even more optimistic NRCan highway rating of 66 mpg(imp.) / 55 mpg(US) is achieved at even lower speeds - about 89 km/h.

The slope of the line sometimes surprises people - but it's important to note the Blackfly test is in top gear like the Auto Bild experiments, not through all the gears like the EPA and GTI graphs show.

Finally, what's the message these charts all send? For the best fuel economy, drive as slow as practical in the tallest gear.